1. Field of the Invention
This invention relates to a method for operating oil-free screw compressors and to an improvement in oil-free screw compressor construction.
2. Description of the Prior Art
Heretofore, the discharge pressure of a single-stage oil-free screw compressor has been limited, for example, to approximately 2-4 kg/cm.sup.2 G maximum due to a temperature rise which occurs at the time of compression. At the present time, some commercialized products have a discharge pressure of about 7.0 kg/cm.sup.2 G thanks to improvements in rotor tooth profile and machining accuracy. In the situation where the discharge pressure is raised to 7.0 kg/cm.sup.2 G, the discharge gas temperature reaches a level of 330.degree.-340.degree. C.
Although the current market has a trend towards demanding compressors with a higher discharge pressure of almost 10 kg/cm.sup.2 G, the discharge gas temperature would rise to 380.degree.-400.degree. C. if the internal pressure ratio were increased to raise the discharge pressure to, for example, 8.5-8.8 kg/cm.sup.2 G. Therefore, it is extremely difficult to prevent rotor contacts due to thermal expansion simply by improving the rotor tooth profile, coupled with difficulty in controlling the gap space of rotor tooth profile during operation.
In order to solve this problem, it is necessary to water- or oil-cool the compressor casing by providing a cooling apparatus, which is, however, reflected by increases in cost and installation floor space.
Thus, under these circumstances, the marked rise in the discharge gas temperature has barred commercialization of air-cooled compressors employing air-cooled casings or water-cooled casings combined with an air-cooling radiator for cooling water to the casings.
On the other hand, especially in an attempt to realize the above-mentioned high discharge pressure, Japanese laid-open patent application No. 60-166785 proposes an oil-free compressor which is designed to raise the discharge pressure without elevating the discharge temperature. For this purpose, part of the discharge gas is returned to the suction side of the compressor, raising the discharge pressure by substantially increasing the suction pressure without varying the internal pressure ratio. More particularly, such discloses that the compressor returns the discharge gas to a position in a gas enclosed zone of the rotor chamber which is partitioned from the suction port and at a position close to the suction port or immediately after suctioning, suppressing increases in the discharge temperature by returning well-cooled gas.
However, although the compressor of the abovementioned patent application, in which part of the discharge gas is returned to the suction side after cooling, is capable of raising the discharge pressure while suppressing the temperature to a low level, such is characterized by the problem that the compressor requires greater power for recompressing part of the discharge gas which is returned to the suction side.